Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Mechanical Alloying And Milling

Synthesis of nanostructured materials by mechanical milling: problems and opportunities

An account is given of the research that has been carried out on mechanical alloying/milling (MA/MM) during the past 25 years. Mechanical alloying, a high energy ball milling process, has established itself as a viable solid state processing route for the synthesis of a variety of equilibrium and non-equilibrium phases and phase mixtures. The process was initially invented for the production of oxide dispersion strengthened (ODS) Ni-base superalloys and later extended to other ODS alloys. The success of MA in producing ODS alloys with better high temperature capabilities in comparison with other processing routes is highlighted. Mechanical alloying has also been successfully used for extending terminal solid solubilities in many commercially important metallic systems. Many high melting intermetallics that are difficult to prepare by conventional processing techniques could be easily synthesised with homogeneous structure and composition by MA. It has also, over the years, proved itself to be superior to rapid solidification processing as a non-equilibrium processing tool. The considerable literature on the synthesis of amorphous, quasicrystalline, and nanocrystalline materials by MA is critically reviewed. The possibility of achieving solid solubility in liquid immiscible systems has made MA a unique process. Reactive milling has opened new avenues for the solid state metallothermic reduction and for the synthesis of nanocrystalline intermetallics and intermetallic matrix composites. Despite numerous efforts, understanding of the process of MA, being far from equilibrium, is far from complete, leaving large scope for further research in this exciting field.

Novel materials synthesis by mechanical alloying/milling

The synthesis and structure of nanocrystalline materials produced by mechanical attrition: A review

The widespread enthusiasm for research on bulk metallic glasses is driven by both a fundamental interest in the structure and properties of disordered materials and their unique promise for structural and functional applications. Unlike the case for crystalline materials, the disordered and nonequilibrium nature of metallic glasses causes their underlying deformation mechanisms to be poorly known. A definite correlation between mechanical behavior and the atomic/electronic structures of metallic glasses has not been established. In this article, I focus on the micromechanisms of mechanical behavior of metallic glasses and present a brief overview of the current understanding of their strength, ductility, and plasticity at the microscopic and atomic scales. The important factors that control the mechanical behavior of metallic glasses are outlined on the basis of recent theoretical and experimental findings. The outstanding issues highlighted in this review are expected to be important for future research ...

Mechanical Behavior of Metallic Glasses: Microscopic Understanding of Strength and Ductility

Amorphous Ni‐Zr powders have been prepared by mechanical alloying from crystalline elemental powders. The glass‐forming range has been determined by x‐ray diffraction, differential scanning calorimetry and saturation magnetization measurements. From 27 to 83 at. % Ni the powders become amorphous. This shows that deep eutectics do not play any role, contrary to amorphization by melt spinning. Crystallization temperatures, crystallization enthalpies, and wave numbers Qp, obtained from x‐ray diffraction investigations, are compared with the data received for rapidly quenched samples. In addition, the effect of the milling intensity on the glass formation has been studied for the first time. If the intensity is too high, crystalline intermetallic phases are formed. On the other hand, the powder needs an extended milling time to become completely amorphous if the milling intensity is too low. Conclusions on the actual temperature of the individual particle during mechanical alloying and on the glass‐forming process are drawn from these results.

Glass-forming range in mechanically alloyed Ni-Zr and the influence of the milling intensity

Quasi‐crystalline Al65Cu20Mn15 powder has been produced by mechanical alloying from crystalline elemental powders. The alloying process has been monitored by x‐ray diffraction, and the resulting product has been characterized by transmission electron microscopy. The quasi‐crystalline phase forms after about 90 h of milling. The crystallization temperature and enthalpy have been determined by differential scanning calorimetry. The results are compared with data for melt‐spun material.

Formation of quasicrystals by mechanical alloying

The formation of quasicrystalline and amorphous powders by mechanical alloying from crystalline, elemental powder has been investigated for AlCuM (M = v, Cr, Fe, Co, Ni) AlMSi (M = Cr, Mn), AlM (M = V, Cr, Mn, Fe, Co), TiNiFeSi, and TiNiV alloy systems. The alloying process has been monitored by X-ray diffraction and differential scanning calorimetry, and the microstructure has been characterized by scanning electron microscopy. Quasicrystalline phases form directly during milling by a solid-state interdiffusion reaction from the crystalline starting materials for AlCuCr and AlCuV. For AlCuFe the formation of the quasicrystalline phase is observed after additional annealing at elevated temperatures. Amorphous phase formation is achieved for the TiNiFeSi, TiNiV, AlMSi (M = Cr, Mn), and AlM (M = V, Fe) alloy systems. For TiNiFeSi and AlCrSi additional annealing results in an amorphous-to-quasicrystal transition. Finally, no alloying during milling has been obtained for binary AlMn, AlCo, and AlCr. Only for AlCr the formation of a quasicrystalline alloy can be observed after isothermal annealing of the layered composite formed during milling. The results are compared with data for rapidly quenched material.

Formation of quasicrystalline and amorphous phases in mechanically alloyed Al-based and TiNi-based alloys

NiTi and FeTi powders prepared by mechanical alloying of elemental crystalline powders in a planetary ball mill have been investigated by X-ray diffraction, differential scanning calorimetry and saturation magnetization measurements. The NiTi powders become completely amorphous in the central composition range. For Ni 70 Ti 30 , the powder consists of an amorphous phase and crystalline intermetallic Ni 3 Ti. The FeTi powders exhibit an amorphous phase from 30 to 70 at.% Fe and residual elemental α-Fe and Ti or crystalline intermetallic FeTi, respectively. The intermetallic phases form directly by milling, since the thermal conditions during extended processing are not sufficient for an in situ crystallization of amorphous powder. The coexistence of amorphous and intermetallic phases indicates that the mechanical alloying does not proceed under metastable equilibrium conditions. Outside the regime of the amorphous phase, mechanical alloying produces markedly extended solid solutions of at least 20 at.% Ti in Ni or Fe. The experimental results are compared with the glass-forming ability predicted by thermodynamic calculations. The amorphization depends strongly on a large negative enthalpy of mixing as a driving force for the interdiffusion reaction during milling. The enhanced solid solubility in both alloy systems can be explained by a small difference of the atomic radii of the individual components and a deformation-induced increase of the free enthalpy of the intermetallic phases.

Synthesis of NiTi and FeTi alloys by mechanical alloying: formation of amorphous phases and extended solid solutions

Binary Co-TM (TM ≡ Mn, Cr, V) alloys have been prepared by mechanical alloying of elemental powders in a planetary ball mill and investigated by X-ray diffraction, differential scanning calorimetry and saturation magnetization measurements. For Co-V, an amorphous phase appears in the central composition range, whereas for Co-Cr and Co-Mn an extended solid solution can be obtained during mechanical alloying. The extent of the glass-forming range is discussed in terms of a thermodynamic approach. The larger the driving force for the interdiffusion reaction the wider the glass-forming range.

K. Urban

Papers

Glass-forming range in mechanically alloyed Ni-Zr and the influence of the milling intensity

Formation of quasicrystals by mechanical alloying

Formation of quasicrystalline and amorphous phases in mechanically alloyed Al-based and TiNi-based alloys

Synthesis of NiTi and FeTi alloys by mechanical alloying: formation of amorphous phases and extended solid solutions

Glass formation and extended solubilities in mechanically alloyed cobalt-transition metal alloys